The present invention relates to an improvement of a drill screw.
Drill screws are self-tapping screws used to hold two members together by means of drill portions which pierce said two members, ridges of a threaded portion following the drill portions threading into holes so formed, and heads following the threaded portion.
The drill portion on a conventional drill screw is shaped as shown in FIG. 4 which is an enlarged view of the drill portion. A chisel 1 to set the position of the drill screw to the work piece is provided at the center tip of this drill screw. This chisel 1 is formed by the intersection of two relative planes 2 (only one of which is shown in FIG. 4) having a specific angle to the axis of the drill screw. The shape of the chisel forms a line perpendicular to the axis. In addition, cutting edges 3 and 4 following off ends 1a and 1b of the chisel 1 extend in a straight line towards outside face 5 of the drill screw, forming a general line of a conical shape centering on the axis of the drill screw.
A conventional drill screw thus described has the following problems. Specifically, when a hole is opened in a work piece by the drill screw, a cutting load is applied to cutting edges 3 and 4. For any given material, if the angles of rakes of these cutting edges are the same, the cutting loads at this time increase as the lengths of the cutting edges 3 and 4 increase. In other words, the cutting load applied to the drill screw is dependent upon the diameter of the drill screw. As a result, there is a limit to the diameter of the drill screw which is manually drillable, and the problem exists that drill screws with diameters greater than or equal to 6.3 mm cannot be manually drilled into a work piece.
In addition, in environments with poor working conditions, it often occurs that the drill screw cannot be held perpendicular to the surface of the work piece, thus causing the drill screw to enter at an angle. In such cases, it is very hard for the chisel 1 to bite the work piece at the specified location. As a result, the reaction force of the work piece causes the tip of the drill screw to slide, causing the drill screw to walk across the surface without fixing the position of the drill hole.